A Dublin man will bring an appeal against the severity of his seven year sentence for killing his partner's son in October.

David Mahon ,46, was found guilty of killing Dean Fitzpatrick ,23, the older brother of missing teenager Amy, on May 26, 2013.

Dean Fitzpatrick received a stab wound to the abdomen outside the apartment his mother, Audrey Fitzpatrick, shared with David Mahon at Burnell Square, Northern Cross, on the Malahide Road in Dublin.

The two-week trial heard that Mahon had been in a relationship with Audrey Fitzpatrick for 12 years by the time her son died.

The State had argued that Mahon was drunk, angry and agitated when he thrust a knife into his stepson with deadly intent. Mahon claimed his death was an accident or possible suicide and that Mr Fitzpatrick had ‘walked into the knife’ while they had been arguing.

06/05/16 Dave Mahon and his wife, Audrey (also the mother of Dean Fitzpatrick) pictured at the Courts of Criminal Justice this today for the third day of jury deliberation in Mr. Mahon's trial for the murder of Dean Fitzpatrick (Image: Colin Keegan)

Mahon was found not guilty of murder but guilty of manslaughter by a jury at the Central Criminal Court. He had pleaded not guilty.

Ms Justice Margaret Heneghan sentenced him to seven years imprisonment on June 13, 2016.

In the Court of Appeal today, Mr Justice George Birmingham fixed October 23 next as the date for Mahon's appeal against sentence.

Mahon was not in court for the case management matter.

Dean Fitzpatrick (Image: Facebook)

At an earlier case management sitting, Mahon, who was represented by Tony Collier Solicitors and two counsel at trial, was granted legal aid for his appeal. The application was uncontested. He had been on legal aid at trial.

Mahon and Audrey Fitzpatrick had moved to Spain’s Costa del Sol with her children, Dean and Amy, in 2004. Mr Mahon worked as an estate agent. He told gardaí that they were millionaires, with eight or nine houses and bars, but that they had spent it all looking for Amy.

The teenager went missing without trace on New Year’s night 2008 as she walked home from a friend’s house along an unlit dirt track. Despite extensive searches, she has never been found.

Dean Fitzpatrick was 17 when his sister vanished. He moved home to Dublin soon after turning 18 and lived with his father, Christopher Fitzpatrick. He met his partner, Sarah O’Rourke, in 2010. They had a son together and lived in Lusk in the north of the county.